Knowledge of wind speed and direction at various altitudes is valuable to aircraft pilots for numerous reasons. Two reasons include fuel consumption and safety. At cruising altitude an aircraft's fuel consumption could be reduced if the pilot was aware of and took advantage of more favorable wind patterns, such as a lower head wind or a higher tail wind. On long haul flights, the selection of favorable winds would result in significant cost savings to the airline.
However, currently there are no effective methods or apparatus for passive profiling atmospheric winds ahead of an aircraft during flight. Active systems using Doppler LIDAR are expensive and have limited range. One method for determining fuel-efficient aircraft altitudes is described in Gary, U.S. Pat. No. 4,474,062. This concept is based on determining the air density at altitudes in the vicinity of the aircraft's present altitude by use of a microwave radiometer to detect the thermal emission from oxygen to indicate the air temperature at a particular altitude. The temperature is then used to determine the air density at that altitude. Since the weight of the aircraft is known during the flight as the fuel is consumed, it is possible to calculate the aircraft weight-to-air density ratio, W/d. This ratio has an optimum value (depending on the type of aircraft) for which the fuel consumption is minimum. The pilot then has the option to change altitude to achieve a more favorable W/d ratio. This method for determining fuel efficient aircraft altitudes is inadequate since it is based only on the determination of the aircraft weight to air density ratio and doesn't take into account the wind speed and direction at the altitude that has the optimum value of W/d. As a result, it may be determined, for example, that the optimum W/d ratio is at an altitude 4000 ft above the present aircraft altitude. If the pilot increases his altitude by 4000 ft and then finds a significant increase in the head winds, this situation could completely negate any fuel efficiency savings due to having a more optimum W/d ratio.
In Gary, U.S. Pat. No. 4,346,595, it is asserted that by knowing the altitude of the local tropopause, it is possible to find efficient flight altitudes by flying about 2000 ft above the tropopause when traveling in a westerly direction and about 2000 ft below the tropopause when going in an easterly direction. The tropopause is generally a broad transitional zone between the troposphere and the stratosphere. From an operational standpoint, it is not practical to use the tropopause as a reference from which to make altitude changes. In addition, the system described in Gary does not take into account local variation in winds that may be present at the new altitudes.
Clear air turbulence (CAT) is a major safety concern for large commercial aircraft. CAT is a weather phenomenon that is due to vertical wind shear in the atmosphere. It usually occurs in temperature inversion layers typically found in the tropopause. CAT results in a rapidly changing airflow over the lift surfaces of the aircraft. Should the forward velocity of the air over the lift surfaces suddenly decrease, the lift will decrease. In this situation, the aircraft may experience a forced descent due to the down flowing air mass and also by an apparent loss in forward air speed. It is thus desirable that an onboard weather system be capable of providing an advanced warning of such wind conditions.
Since the conditions that result in clear air turbulence are not visually apparent nor are they generally detectable by active sensors such as radar, there have been a number of attempts to detect wind shear and clear air turbulence conditions by passive detectors. In particular, attempts have been made to sense air temperature gradients, which are associated with air turbulence, by detecting the radiation emanating from the atmosphere ahead of the aircraft in the infrared and microwave spectral regions. The intensity of the detected radiation varies with the atmospheric temperatures along the line of sight of the detector. Typically these passive systems use a radiometer to measure the thermal radiation from one of the atmospheric gases such as carbon dioxide (CO.sub.2), oxygen (O.sub.2) or water vapor (H.sub.2 O) to determine changes in the spatial temperature profile in front of the aircraft. Examples of such approaches based on the infrared emission of CO.sub.2 are provided in U.S. Pat. Nos. 3,475,963, 3,735,136, 3,780,293, 3,935,460, 4,266,130, 4,427,306, 4,937,447, 4,965,572, 4,965,573, 5,105,191, 5,276,326 and 5,285,070. Other approaches determine atmospheric temperature by measuring the microwave emission from O.sub.2 as described in U.S. Pat. Nos. 3,359,557, 3,380,055, 4,346,595, and 5,117,689. Systems for measuring atmospheric temperature based on infrared emission from H.sub.2 O are described in U.S. Pat. No. 4,266.130 and in the paper by Kuhn et al, "Clear Air Turbulence: Detection by Infrared Observations of Water Vapor" in Science, Vol. 196, p.1099, (1977). In addition, there have been several papers written describing these types of passive infrared systems including: S. M. Norman and N. H. Macoy, "Remote Detection of Clear Air Turbulence by Means of an Airborne Infrared System" AJAA Paper No. 65-459 presented at the AIAA Second Annual Meeting, San Francisco, Calif., Jul. 26-29, 1965; and R. W. Astheimer, "The Remote Detection of Clear Air Turbulence by Infrared Radiation" in Applied Optics Vol. 9, No. 8, p.1789 (1970). In U.S. Pat. No. 4,346,595, Gary describes a microwave radiometer for determining air temperatures in the atmosphere at ranges of about 3 km from the aircraft for the purpose of detecting the height of the tropopause and the presence of temperature inversions. He teaches that by flying the aircraft above or below the tropopause or temperature inversion layer, it is possible to avoid CAT. Since the effective range of the microwave radiometer is relatively short, the system doesn't provide sufficient warning time for the aircraft to avoid the CAT condition. The present invention has detection ranges on the order of 100 km which will allow time for the aircraft to change altitude to avoid CAT.
A number of the above systems were not successful or were only partially successful because they were based solely on the measurement of atmospheric temperature in order to predict the presence of turbulence. A more reliable indication of atmospheric turbulence can be realized by determining the Richardson number, Ri. The use of the Richardson number to determine the stability of the atmosphere is well known in meteorology (see, for example, D. Djuric, Weather Analysis, Prentice Hall, Englewood Cliffs, N.J., 1994, p.64). In the present invention, the Richardson number is used to indicate the probability of CAT. In U.S. Pat. No. 5,117,689, Gary discussed the correlation of the reciprocal of the Richardson number with the occurrence of CAT conditions. The Richardson number, Ri, contains two components: (1) the vertical lapse rate of potential temperature and (2) the wind shear which is related to the horizontal temperature gradient. A number of the prior art discussions measure the vertical temperature lapse rate. Gary used the inertial navigation system (INS) to measure the East-West and North-South components of the wind (the wind shear) along with a microwave radiometer to measure the air temperature vertical lapse rate. This information is then used to calculate the Richardson number or its reciprocal. The deficiency of the system described in this patent (U S. Pat. No. 5,117,689) is that it determines the Richardson number at relatively close ranges (less than 3 km) and therefore does not provide advance warning of the CAT condition and that it measures the wind shear only at the aircraft.
Previous approaches for the determination of the range and probability of CAT can be summarized as follows:
U.S. Pat. No. 5,276,326 to Philpott determines turbulence as a function of temperature vs. range through the analysis of infrared radiometer signals at two or more discrete wavelengths. The temperature associated with a given range as a function of wavelength is then derived through a matrix inversion process. This transition is difficult and requires noise and error free input data to yield valid results. The present invention overcomes this difficulty by using only one wavelength. Gary overcomes the multiple wavelength difficulty in U.S. Pat. No. 4,346,595 by measuring effective temperature and range at a single wavelength, however no attempt is made to determine the probability of clear air turbulence using the Richardson number (Ri). In U.S. Pat. No. 5,117,689, Gary teaches the significance of the Richardson number in CAT prediction but does not suggest a method to derive Ri directly from radiometric measurements of horizontal and vertical temperature lapse rates obtained by combining azimuth and elevation scanning with the aircraft motion to produce a temperature map.